The present invention relates to an apparatus for use with wire bonding machines, of the type used to bond fine conductive wires to miniature electronic devices, such as integrated and hybrid micro-circuit chips. More particularly, the invention relates to an automatic wire de-spooler apparatus for paying out wire for use by such bonding machines.
B. Description of Background Art
Miniature electronic circuits, or micro-circuits are used in vast quantities, in a wide variety of consumer, commercial, industrial and military apparatus. The majority of such micro-circuits are of a type referred to as integrated circuits. Integrated circuits contain a large number of active circuit elements such as transistors, and passive elements such as resistors and capacitors. In semiconductor integrated circuits, conductive paths between circuit elements on a semiconductor substrate are formed by selectively etching the substrate. In hybrid micro-circuits, circuit elements mounted on a ceramic substrate are usually interconnected, typically by conductive ink paths on the substrate.
The functional portions of integrated circuits are typically in the form of very small, rectangular-shaped chips, ranging in size from 0.025 inch to 0.200 inch or more on a side. Input connections to integrated circuit chips are often made by bonding a very fine wire to conductive pads on the chips, the other end of each wire being bonded to a conductive terminal that is sufficiently large and robust to be inserted into a printed circuit board and soldered to conductors on the board.
Typically, bonding wire used to interconnect the pads of a semiconductor chip to terminals of a package containing the chip is made of aluminum or gold, and is quite fine, having a diameter of about 1 mil. (0.001 inch). This wire must be bonded to small, typically rectangular-shaped, integrated circuit pads a few mils on a side.
The most common method of interconnecting wires between semiconductor chip pads and external terminals is to form a weld or bond at each end of a conducting wire. The bonds are formed by the application of heat, ultrasonic energy, or a combination of both. To form such bonds, the free end of a length of bonding wire is placed in contact with a pad. Then the tip of an ultrasonic transducer is pressed against the wire, and energized with ultrasonic energy for a short time interval, welding the wire to the pad. The unbonded length of wire is then moved to other pads, and bonded thereto by the same process. After the last bond in a series of bonds has been thus formed, the wire is severed near the last bond.
Typical wire bonding machines used for ultrasonic welding of wires to micro-circuit pads include an elongated, vertically disposed, force-applying member or "tool." The tool is connected at the upper end thereof to a source of ultrasonic energy, such as a piezoelectric transducer connected to an electrical energy source alternating at an ultrasonic frequency. Usually, the tool is connected to the transducer through a tapered horn structure that matches the acoustic input impedance of the small tool to the output impedance of the larger transducer.
Ultrasonic bonding tools used to bond wires to microcircuit pads generally have a flat lower working face adapted to press a bonding wire into contact with a pad, while ultrasonic energy is applied through the tool to the wire to form an ultrasonic weld. This working face is usually quite small, typically having a rectangular shape only about a few mils along a side. The working face must be quite small to permit bonding to small micro-circuit pads, without contacting adjacent circuit elements. Typically, this is done while viewing the pad and tool tip in a stereo microscope.
In most wire bonding machines, the bonding tool is adapted to manipulate bonding wire over a pad, prior to performing the bonding operation. Such bonding tools may include an upwardly angled lower face rearward of the working face, and a generally vertically disposed rear face. An angled bore or guide hole having an entrance aperture in the rear face and an exit aperture in the angled lower face permit bonding wire from a spool mounted upward and rearward of the tool to be paid out through the exit aperture of the angled lower face. Typically, a remotely actuable clamp located rearward of the guide hole entrance and movable with the tool is used to feed bonding wire through the guide hole of the tool.
The clamp used to effect movement of wire through the guide hole of a bonding tool usually consists of a pair of jaws that may be closed to grip the wire, or opened to allow free travel of the wire. Generally, such clamps may be moved toward and away from the guide hole entrance, typically on a line of movement which coincides with the axis of the guide hole. To feed wire through the guide hole, the jaws of the clamp are first opened, and the clamp then moved away from the guide hole. The jaws are then closed to grip the wire, and then moved towards the guide hole, thus feeding wire through the guide hole.
In wire bonding machines of the type just described, the machine is used to move the bonding tool to the proper position to bond wire to a pad, feed wire out through the guide hole exit aperture, move the tool to another pad and form another bond. In this manner, any desired number of pads or other elements of a circuit can be connected together, in a procedure referred to a "stitch" bonding. After the last bond in a series of bonds has been made, the wire must be severed, to permit making other, unconnected bonds. Oftentimes, the bonding tool itself is utilized to sever the bonding wire.
The bonding machines described above are often referred to a "wedge bonding bonders," owing to the shape of the ultrasonic tool tip used to make bonds. Another type of bonding machine uses a fine wire, usually made of gold, that protrudes through a capillary tube and is melted with a miniature torch to form a bond consisting of a fused ball at the end of the wire.
Both wedge bonding and capillary ball bonding operations require that the bonding wire be supplied to the tip of the bonding tool with very little drag or tension. Even a small amount of drag can make the "tail," or length of wire at a bond site to be too short, resulting in a bond of insufficient strength. Too much drag can also result in loops between bonds that are too short. Excessive drag can even result in wire breakage.
From the discussion above, it should be evident that it is desirable to provide sufficient slack in the wire supply of wire bonding machines to ensure that minimum drag is placed on the wire. One prior art approach to maintaining slack in wire supplied to bonding machine uses helical loops of wire paid axially off the end of a stationary spool, similar to the operation of a spinning reel used for fishing. This method of paying out wire, sometimes referred to as "ballooning," has the disadvantage of imposing a torsion on wire paid out, causing the wire to pick up a twist. Point-to-point connections made with wire twisted in this manner tends to bend away from a vertical plane normal to the horizontal plane containing a microcircuit substrate and conductive pads, a condition referred to variously as "sweeping" or "dog-legging." This condition is undesirable, since wire sweep or dog-legging can degrade bond strength, and if sufficiently large, cause bonding loops to short out against one another or even against the microcircuit itself. Another prior art method of providing slack in the wire supplied to a wire bonder uses a motor-driven spool. In this type of device, the motor receives pre-programmed signals causing it to rotate intermittently in controlled increments. The present invention was conceived of to provide an improved means for supplying wire to wire bonding machines, while maintaining a precisely controlled amount of slack in the wire.